T. Scopigno

Relaxation Processes in Harmonic Glasses

A relaxation process, with the associated phenomenology of sound attenuation and sound velocity dispersion, is found in a simulated harmonic Lennard-Jones glass. We propose to identify this process with the so-called microscopic (or, instantaneous) relaxation process observed in real glasses and supercooled liquids. A model based on the memory function approach accounts for the observation and allows one to relate to each other (1) the characteristic time and strength of this process, (2) the low frequency limit of the dynamic structure factor of the glass, and (3) the high frequency sound attenuation coefficient, with its observed quadratic dependence on the momentum transfer.

Density fluctuations in molten lithium: inelastic x-ray scattering study

New inelastic x-ray scattering experiments have been performed on liquid lithium at two different temperatures: T = 475 K (slightly above the melting point) and 600 K. Taking advantage of the absence of any kinematical restriction and incoherent contribution, and pushing the instrumental resolution up to 1.5 meV, it was possible to perform an accurate investigation of the dynamic structure factor S(Q, E)in the wavevector range from 1 to 110 nm −1 .F orQ smaller than Qm � 25 nm −1 , the position of the main peak of the static structure factor, a detailed analysis of the lineshapes shows that any picture of the relaxation mechanisms based on a simple viscoelastic model must be abandoned. All the spectral features can instead satisfactorily be accounted for by including both fast and slow relaxation processes. The physical origin of the slow relaxation is associated with the structural rearrangement, while the local nature of the fast one is extensively discussed. At larger Q-values, a gradual crossover from the strongly correlated to single-particle dynamics occurs, with an important weighting provided by quantum effects.New inelastic X-ray scattering experiments have been performed in liquid lithium at two different temperatures: T=475 K (slightly above the melting point) and 600 K. Taking advantage of the absence of any kinematical restriction and incoherent contribution, and pushing the instrumental resolution up to 1.5 meV, it was possible to perform an accurate investigation of the dynamic structure factor S(Q,E) in the wavevector range from 1 to 110 nm^-1. For Q smaller than Q_m=25 nm^-1, the position of the main peak of the static structure factor, a detailed analysis of the lineshapes shows that any picture of the relaxation mechanisms based on a simple viscoelastic model must be abandoned. All the spectral features can instead satisfactorily be accounted for by including both fast and slow relaxation processes. The physical origin of the slow relaxation is associated to the structural rearrangement, while the local nature of the fast one is extensively discussed. At larger Q values a gradual crossover from the strongly correlated to single particle dynamics occurs, with an important weight provided by quantum effects.

Evidence of two viscous relaxation processes in the collective dynamics of liquid lithium.

New inelastic x-ray scattering experiments have been performed on liquid lithium in a wide wave vector range. With respect to the previous measurements, the instrumental resolution, improved up to 1. 5 meV, allows one to accurately investigate the dynamical processes determining the observed shape of the dynamic structure factor S(Q, omega). A detailed analysis of the line shapes shows the coexistence of relaxation processes with both slow and fast characteristic time scales, and therefore shows that pictures of the relaxation mechanisms based on a simple viscoelastic model must be abandoned.

Nature of the short wavelength excitations in vitreous silica: An X-ray Brillouin scattering study

The dynamical structure factor (S(Q,E)) of vitreous silica has been measured by Inelastic X-ray Scattering varying the exchanged wavevector (Q) at fixed exchanged energy (E) - an experimental procedure that, contrary to the usual one at constant Q, provides spectra with much better identified inelastic features. This allows the first direct evidence of Brillouin peaks in the S(Q,E) of SiO_2 at energies above the Boson Peak (BP) energy, a finding that excludes the possibility that the BP marks the transition from propagating to localised dynamics in glasses.

Acoustic nature of the boson peak in vitreous silica

New temperature-dependent inelastic X-ray scattering (IXS) and Raman scattering (RS) data are compared with each other and with existing inelastic neutron scattering (INS) data in vitreous silica, in the 300± 1775K region. The IXS data show collective propagating excitations up to Q ˆ 3:5nm i 1 . The temperature behaviour of the excitations at Q ˆ 1:6nm i 1 matches that of the boson peak found in INS and RS. This supports the fact that the acoustic origin of the excess of vibrational states gives rise to the boson peak in this glass.

Collective dynamics of liquid aluminum probed by inelastic x-ray scattering

An inelastic X-ray scattering experiment has been performed in liquid aluminum with the purpose of studying the collective excitations at wavevectors below the first sharp diffraction peak. The high instrumental resolution (up to 1.5 meV) allows an accurate investigation of the dynamical processes in this liquid metal on the basis of a generalized hydrodynamics framework. The outcoming results confirm the presence of a viscosity relaxation scenario ruled by a two timescale mechanism, as recently found in liquid lithium.

Phonon-like and single-particle dynamics in liquid lithium

The dynamic structure factor, S(Q,E), of liquid lithium (T=475 K) has been determined by inelastic x-ray scattering (IXS) in the momentum transfer region (Q = 1.4-110 nm-1). These data allow to observe how, in a simple liquid, a phonon-like collective mode evolves towards the single particle dynamics. As a function of Q, one finds: i) at low Qs, a sound mode with a positive dispersion of the sound velocity, ii) at intermediate Qs, excitations whose energy oscillates similarly to phonons in the crystal Brillouin zones, and iii) at high Qs, the S(Q,E) approaches a Gaussian shape, indicating that the single particle dynamics has been reached.

Observation of Umklapp processes in noncrystalline materials

Umklapp processes are k nown to exist i n cristalline materials, where they c ontrol important properties s uch a s thermal conductivity, heat capacity and electrical conductivity. In this work we report the provocative observation of Umklapp processes in a non-periodical system, namely liquid Lithium. The lack of a well defined periodicity seems then no t t o p revent t he ex istence of t hese scattering p rocesses mechanisms provided that the local order of the systems i.e. the maxima of the static structure factor supply the equivalent of a reciprocal lattice vector in the case of cristalline materials.

Elastic Constant Inhomogeneity and the Broadening of the Dynamic Structure Factor in One-Dimensional Disordered Systems

We propose an explanation for the quadratic dependence on the momentum

Inelastic X-ray scattering determination of the dynamic structure factor of liquid lithium

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